Wavelength locker for simultaneous control of multiple dense wavelength division multiplexing transmitters

ABSTRACT

An apparatus comprising a plurality of optical transmitters coupled to a fiber, a signal generator coupled to the optical transmitters and configured to provide a single pilot tone to the optical transmitters, and a processor positioned within a feedback loop between the fiber and the optical transmitters, the processor configured to adjust a wavelength for each of the optical transmitters to lock the wavelengths. An apparatus comprising at least one processor configured to implement a method comprising receiving an optical signal comprising a pilot tone, detecting an amplitude and a phase of the pilot tone, calculating a quadrature term using the amplitude and the phase, and wavelength locking the optical signal using the quadrature term.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Conventional wavelength division multiplexing (WDM) transmitter arraysmay employ semiconductor lasers as optical transmitters. The performanceof WDM systems requires signal integrity from the laser sources. Thewavelengths of the output signals from the WDM laser arrays may vary dueto manufacturing process variations, device age, temperature, or otherfactors. Individual WDM lasers may have different electrical, optical,and/or thermal properties that also affect the lasers' wavelengths.Wavelength locking can facilitate signal integrity, but is difficult todo when the lasers' wavelengths vary.

One approach to provide wavelength locking has been to employ a feedbacksystem to compare actual laser output wavelength to the target laseroutput wavelength. Laser output can then be adjusted to correct fordeviations. In some applications, a wavelength locker is used for eachlaser in an array. As the number of individual optical transmittersincreases, the complexity and cost for wavelength locking may alsoincrease. What is needed is a way to provide efficient andcost-effective wavelength locking.

SUMMARY

In an embodiment, the disclosure includes an apparatus comprising aplurality of optical transmitters coupled to a fiber, a signal generatorcoupled to the optical transmitters and configured to provide a singlepilot tone to the optical transmitters, and a processor positionedwithin a feedback loop between the fiber and the optical transmitters,the processor configured to adjust a wavelength for each of the opticaltransmitters to lock the wavelengths.

In an embodiment, the disclosure includes an apparatus comprising atleast one processor configured to implement a method comprisingreceiving an optical signal comprising a pilot tone, detecting anamplitude and a phase of the pilot tone, calculating a quadrature termusing the amplitude and the phase, and wavelength locking the opticalsignal using the quadrature term.

In an embodiment, the disclosure includes a method comprising receivingan optical signal generated by an optical transmitter, the opticalsignal having a spectral shape, determining a peak of the opticalsignal, locking onto the peak, and determining whether an output fromthe optical transmitter needs to be adjusted based on the locking.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a graph of an embodiment of a plurality of transmissionspectra from an etalon.

FIG. 2 is a graph of an embodiment of a plurality of optical intensityspectra of different modulated signals.

FIG. 3 is a graph of an embodiment of a plurality of etalon transmissionspectra of different modulated signals.

FIG. 4 is a graph of an embodiment of a plurality of etalon transmissionspectra of 10G NRZ signals under different thermal chirp conditions.

FIG. 5 is a top plan view of an embodiment of a WDM laser transmitter.

FIG. 6 is a schematic diagram of an embodiment of a WDM laser wavelengthlocking apparatus.

FIG. 7 is a flowchart of an embodiment of a WDM laser wavelength lockingmethod.

FIG. 8 is a graph of an embodiment of a plurality of frequency responsesof a quadrature component under different chirp conditions.

FIG. 9 is another graph of an embodiment of a plurality of frequencyresponses of a quadrature component with different modulated signals.

FIG. 10 is a schematic diagram of an embodiment of a general-purposecomputer system.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Conventional wavelength locking schemes used in WDM applications may usea wavelength locker for each wavelength. For example, for wavelengthspacing of about 100 Gigahertz (GHz) or less, a wavelength locker may beused for each laser. For wavelength locking, a reference signal ismeasured, as well as a signal transmitted through an etalon. These twosignal intensities are then compared to determine the deviation from atarget wavelength. Typically, the wavelength lockers are designed tolock at the short-frequency side of the etalon transmission peaks. Thelocking point may be determined as a distance equal to about 20 percentof the free spectral range from its peak. This point is defined by theintersection of the two spectra at the upward slope of the etalonspectra.

FIG. 1 shows spectra obtained from a 50 GHz free spectral range etalonwavelength locker and a reference signal. The straight line is thereference signal, while the periodical line is from the signaltransmitted through the etalon. These spectra are obtained by scanning alaser with a sufficiently narrow line width (narrower than the linewidth of the etalon) and plotting the received signals as a function ofthe laser wavelength. If the reference signal is equal to thetransmitted signal in strength, the wavelength is considered to belocked. Otherwise, it is considered to be off from its target. The signof the difference in signal intensity seen by the two detectors tellsthe direction of the deviation so that appropriate actions can be takento correct the wavelength back to its target. In the case illustrated inFIG. 1, the locking point may be determined to be the point where theperiodic line crosses the straight line with an upward slope. That is,the locking point may be at about −50 GHz, about zero GHz, or about 50GHz, each being essentially the same, based on the repetitive nature ofthe signal.

In another scheme, multiple WDM transmitters may share one wavelengthlocker. In this scheme, a mechanism is needed to choose one wavelengthat a time for detection. There are a number of methods used toaccomplish the task. In Time Division Multiplexing (TDM), a tunableoptical filter may be used with the wavelength locker. In apre-determined time window, the locker may be tuned to one selectedwavelength using the filter. The selected wavelength can then becorrected in a manner identical to that of the dedicated wavelengthlocker described above. By scanning the filter across multiplewavelengths, the wavelength of each transmitter may be determined,relative to the target, one wavelength channel at a time. The wavelengthof each transmitter can then be corrected if there is any deviation fromthe target. Since the wavelength of a transmitter may drift slowly (e.g.on the order of hours or days), this filter-based TDM approach may besufficiently reliable and is commonly used in the transport industry forcentralized wavelength locking.

An alternative TDM approach is to introduce a dither signal to thewavelength selected to be stabilized. The dither signal has apre-selected frequency, and is added to the normal data beingtransported by the WDM transmitter. The frequency is typically betweenabout several tens of kilohertz (kHz) to about several hundreds of kHz.The same etalon unit can be used to lock the selected wavelength thathas the dither applied by only considering the signal componentcorresponding to the dither frequency. If the same edge-lockingmechanism described above is used, one can determine the wavelengthdeviation by comparing the two components of the reference and etalonsignal at the dither frequency. If the etalon signal is greater thanthat of the reference signal, the output wavelength may be shorter thanthe desired wavelength and the laser transmitter must be adjusted to alonger wavelength. Otherwise, the output wavelength may be longer thanthe desired wavelength and must be adjusted in the opposite direction.If the two signals are equal, the transmitter may be operating at thedesired wavelength. A control loop, such as a digital or analog controlloop, can be built based on the ratio of the two signals (also referredto as the α parameter) to stabilize the wavelength.

Frequency division multiplexing (FDM) can also be used with dithersignals. In the FDM case, a dither with a different frequency isintroduced to each WDM transmitter. For example, each transmitter n(n=1, 2 . . . N) may use a dither frequency f_(n), where N is aninteger. The optical signals from the N transmitters arewavelength-division multiplexed onto a single fiber and then detected.The detected signals are then broadcasted to N channels, each of whichmeasures the component corresponding to its corresponding ditherfrequency using the proper choice of electronic filters. Thus, one maysimultaneously obtain N cc parameters, which may each indicate thedeviation of the wavelength from its target. In the case of TDM, thesame dither frequency is used for all N transmitters but is applied onetransmitter at a time, as in the case of the DC approach. For instance,in time slot t_(n), the dither is applied to transmitter n and thewavelength locker only detects wavelength α_(n). In time slot, t_(n)+1,the process is repeated for transmitter n+1 and so forth.

Both the TDM and FDM based methods are found to be cost-effective forcentralized wavelength stabilization in WDM applications. However, theuse of filters for wavelength locking via TDM has disadvantages. Forinstance, the TDM based method may not be cost effective to implement. Atypical tunable optical filter for 50 GHz spacing may cost more than awavelength locker. In addition, this approach to wavelength locking haslimitations. Further, the TDM based method may only be used with theedge-locking method which may require that the optical spectra of all ofthe transmitters be nearly identical, as described in more detail below.If the spectra vary from transmitter to transmitter, such as in the caseof directly modulated lasers and/or different modulation formats/rates,the edge-locking approach may introduce large variations in the lockingpoints.

FIG. 2 shows four spectra obtained with four different modulations: acontinuous wave (CW) source, a 10 GHz non-return-to-zero (NRZ)modulation obtained with a directly modulated laser (DML), a 10 GHz NRZmodulation obtained with an electro-absorption modulated laser (EML),and 40 GHz Differential Phase-Shift Keying (DQPSK). Using the samemodulation at 10 GHz, different lasers using the same modulation mayhave different spectral shapes. For example, the spectrum of a 10 GHzNRZ modulation obtained with the DML has a broader spectrum, due tochirping, than the EML. If the same edge-locking method is used as shownin FIG. 2, the locking point will vary substantially, depending upon themodulated line-width.

FIG. 3 depicts the different intersection points with the referencesignal (shown as a straight line) for the different modulations shown inFIG. 2. In FIG. 3, the etalon spectrum is a convolution of the etalonspectrum with the modulated line-width. Consequently, at the samelocking point, as defined by an intensity equal to that of the referencesignal, the corresponding wavelength varies significantly. For thetransmitter in FIG. 2 with 40 GHz DQPSK modulation, the detectedwavelength is off to the short frequency by as much as about 5 GHz,compared to an offset of about 0.5 GHz in the case of NRZ modulationusing EML.

The dither approach using a single wavelength locker via either TDM orFDM has additional problems if the edge-locking method is used. Thespectral variation can introduce an uncertainty in the locking point,and the dither itself may interfere with the locking mechanism. Thedither commonly applied to the bias current of a semiconductor laser(e.g. a distributed feedback laser) will introduce additional adiabaticand thermal chirp to the transmitter.

FIG. 4 shows transmission spectra of the etalon signal with chirpsinduced by different dither signals. A large spectral distortion isapparent at the left side of the transmission peak where the traditionallocking point would be chosen by the edge-locking approach. As shown,the distortion in some cases can be substantially large where the risingedge is no longer monotonic, making the edge locking method unusable.This large spectral distortion is a result of the thermal chirp whichdominates the overall chirp caused by the dither. Thermal chirp dependsstrongly on factors such as the transmitter packaging, its geometry, itsthermal contact, and its cooling efficiency. These parameters may varyfrom manufacturer to manufacturer. In the case where a dedicated lockeris used for each transmitter, the undesirable spectral distortion can beminimized by choosing a dither frequency that has reduced thermal chirp.However, in a centralized locking scheme, one may not have the freedomto do so because the lasers to be stabilized may be manufactured bydifferent vendors with different technologies. As a result, a ditherfrequency that may be good for one laser may lead to spectral distortionin the others and vice-versa, which may not be practical.

Disclosed herein is an apparatus and method that provide wavelengthlocking of optical laser transmitters in a laser transmitter system,such as a WDM laser transmitter. In an embodiment, the apparatus andmethod facilitate the determination of a locking point of laser opticaltransmitters, based on the peak of the spectrum of the laser. Thedetermination of the locking point may not be significantly affected byfactors such as variations in laser spectral line shape, modulationformat, thermal chirp, adiabatic chirp, data rate, device manufacturingprocess variations, device age, temperature, or other factors. Inaddition, a single pilot signal generator is employed, therebyminimizing cost and complexity. By detecting both the amplitude andphase of the pilot signal that is combined with the signal of the laseroptical transmitter, rather than just the power of the pilot signal,improved locking point determination is possible. In addition, thequadrature component of a first derivative of the transmission functionof the etalon filter used in the apparatus and method may provideeffective wavelength locking.

The laser transmitter system may comprise a plurality of opticaltransmitters, such as lasers, photodiodes, other devices configured totransmit electromagnetic waves at optical wavelengths, or combinationsthereof. The optical wavelengths may comprise at least a portion of thevisible wavelength range, infrared wavelength range, ultraviolet (UV)wavelength range, or other optical wavelength ranges. In an embodiment,the optical transmitters may be discrete transmitter units, which may becoupled to one another. For example, the discrete optical transmittersmay be mounted in an array arrangement on a chip, card, or opticalplatform. The optical transmitters may also be coupled to an opticalcoupler, such as a multiplexer, which may be configured to combine theoutputs from the different optical transmitters into a single output.The outputs from the different optical transmitters may have differentwavelengths and the output from the optical coupler may comprise thedifferent wavelengths of the optical transmitters. The optical couplermay be positioned on the same or different chip, card, or opticalplatform. The optical coupler may be coupled to the optical transmittersvia a plurality of fibers or waveguides, and may also be coupled to anoutput via an additional fiber or waveguide. Additionally, the lasertransmitter system may comprise a signal generator and a wavelengthlocking apparatus, which may be coupled to the optical transmitters andthe optical coupler. The signal generator may provide a pilot signalonto the output of any of the optical transmitters or the opticalcoupler, and the wavelength locking apparatus may lock the wavelength ofan optical transmitter using the pilot tone, as described below. In analternative embodiment, at least some of the components of the lasertransmitter system may be integrated into a chip, such as a planarlightwave circuit (PLC).

FIG. 5 illustrates an embodiment of a WDM laser transmitter 100. The WDMlaser transmitter 100 may comprise a platform 110, a laser dice 120, astep 130, a plurality of first channels 140, an arrayed waveguide (AWG)150, and optionally a second channel 155. The WDM laser transmitter 100may also comprise or may be coupled to a fiber 160. These components maybe configured according to known configurations, such as a hybridintegration configuration or a monolithic configuration. The WDM lasertransmitter 100 may emit a plurality of distinct Dense WDM (DWDM)channels, as described in the International Telecommunication UnionTelecommunication Standardization Sector (ITU-T) G.694.1, and/or coarseWDM (CWDM) channels, as described in the ITU-T G.694.2. As such, the WDMlaser transmitter 100 may be suitable for use in backbone and/or accessoptical networks.

In an embodiment, the platform 110 may be configured to accommodate andintegrate the components of the WDM laser transmitter 100. Specifically,the platform 110 may comprise at least one material that integrates,bonds, and/or supports the components of the WDM laser transmitter 100.The platform 110 may be produced using a deposition process, forinstance on a chip or substrate. Further, the platform 110 may comprisea plurality of layers at different sites, which may be produced usingdeposition and/or etching. The layers may bind together other componentsof the WDM transmitter 100, such as the first channels 140, the AWG 150,and the second channel 155. Additionally, the layers may mount orsupport a component of the WDM laser transmitter 100, such as the laserdice 120. In an embodiment, the platform 110 may comprise a thin filmlayer of a dielectric material, such as silicon dioxide (SiO₂), whichmay be deposited on a substrate using chemical deposition, such aschemical solution deposition (CSD), chemical vapor deposition (CVD), andplasma-enhanced CVD (PECVD). Alternatively, the film layer may bedeposited using physical deposition, such as thermal evaporation,sputtering, pulsed laser deposition, or cathodic arc deposition(arc-PVD). Other deposition processes also may be used, includingreactive sputtering, molecular beam epitaxy (MBE), metalorganic vaporphase epitaxy (MOVPE), topotaxy, or any other suitable process. The thinfilm layer also may be etched at some regions of the platform 110 usingwet chemical etching or dry plasma etching. The thin film layer may havea thickness less than about one mm, such as about ten micrometers.

The laser dice 120 may be the light emitting components of the WDM lasertransmitter 100. The laser dice 120 may be coupled to the platform 110and comprise a plurality of integrated semiconductor lasers, which maybe configured in an array. For instance, an array of semiconductorlasers may be produced by depositing a lasing material, such as indiumphosphide (InP) or gallium arsenide (GaAs), at a plurality of alignedsites on a chip. Alternatively, the lasing material may be added to thechip using chemical or electrochemical doping. The laser dice 120 may belaser diodes, heterostructure lasers, quantum well lasers, quantumcascade lasers, distributed feedback (DFB) lasers, combinations thereof,or other. The laser dice 120 may be configured to transmit a pluralityof light waves substantially in the same direction, e.g., from the sameside of the array. The laser dice 120 also may be configured to transmitthe light waves at a plurality of wavelengths that span a bandwidth. Inan embodiment, the wavelengths may be spaced by about the same value,where the difference between the values of any two wavelengths may beabout the same. In an embodiment, the laser dice 120 may be coupled tothe platform 110 via bonding.

In an embodiment, the laser dice 120 may be accommodated by the step 130and the platform 110. For instance, the step 130 may be positioned atone edge of the platform 110 and coupled to the laser dice 120. The step130 may comprise a layer of the platform 110, which may be produced viaetching or deposition, and as such may comprise the same material as theplatform 110, e.g. SiO₂. The step 130 also may be coupled to externalelectrical components, which may be used to operate and/or modulate theWDM laser transmitter 100, as described below.

The light emitted from the laser dice 120 may be transported to othercomponents of the WDM laser transmitter 100 via the first channels 140.As such, the first channels 140 may be coupled to the laser dice 120 andthe AWG 150 and may be aligned with the laser dice 120. The firstchannels 140 may comprise a plurality of waveguides, which may beconfigured to transfer the light from the laser dice 120 to the AWG 150.The waveguides may be dielectric waveguides, which may comprise adielectric material that has a higher permittivity or dielectricconstant than the surrounding platform 110. For example, the firstchannels 140 may be produced by depositing a layer of higher indexmaterial on the platform 110, etching the surrounding areas, and thendepositing the same material as the platform 110 until the depositedmaterial forms the upper cladding. Such a process may produce the samecladding material all around the channels 140. Thus, the light waves maybe guided through the first channels 140 by total internal reflectionfrom the laser dice 120 to the AWG 150.

The light waves transported by the first channels 140 may be combinedinto a single light wave at the AWG 150, and hence transmitted from theWDM laser transmitter 100. Accordingly, the AWG 150 may be coupled tothe first channels 140 and the second channel 155. The AWG 150 may be anoptical multiplex (MUX) configured to combine a plurality of light wavesfrom the first channels 140 into a combined light wave that propagatesin the second channel 155. For instance, the AWG 150 may comprise aplurality of grating waveguides, which may have different lengths, whereeach two adjacent grating waveguides may have about the same lengthdifference. The light waves may correspond to the individualsemiconductor lasers in the laser dice 120, where each light wave mayhave a different wavelength. The light waves may propagate through thegrating waveguides, undergo a change of phase due to the lengthdifference between the adjacent grating waveguides, and constructivelyinterfere into the combined light wave at an output of the AWG 150.Hence, the combined light wave may comprise all the wavelengths of theindividual light waves. The grating waveguides may be dielectricgratings waveguides, which may comprise the same material as the firstchannels 140, and may be produced using a process similar to the processused to produce the first channels 140.

The combined light may be transmitted from the WDM laser transmitter 100using the second channel 155 and the fiber 160. The second channel 155may comprise a dielectric waveguide, similar to the first channels 140.The second channel 155 may be coupled to the AWG 150 and the fiber 160,and as such may guide the combined light from the AWG 150 to the fiber160. The second channel 155 may be produced using a process similar tothe process used to produce the first channels 140. In an embodiment,the first channels 140, the AWG 150, and the second channel 155 may bepositioned in the same layer of the platform 110 and may be aligned withthe laser dice 120.

In an embodiment, the fiber 160 may be an optical fiber, which may beused to transport the combined light wave from the WDM laser transmitter100 to an optical system, such as an optical telecommunications systemor an optical network. Specifically, the fiber 160 may be used totransport WDM signals, such as the DWDM and/or CWDM signals describedabove. The fiber 160 may be a standard single mode fibers (SMFs) asdefined in ITU-T standard G.652, dispersion shifted SMF as defined inITU-T standard G.653, cut-off shifted SMF as defined in ITU-T standardG.654, non-zero dispersion shifted SMF as defined in ITU-T standardG.655, wideband non-zero dispersion shifted SMF as defined in ITU-Tstandard G.656, multimodal fiber, or any other type of fiber. All of thestandards described herein are incorporated herein by reference.

FIG. 6 illustrates an embodiment of a wavelength locking system 200. Thewavelength locking system 200 comprises the WDM laser transmitter 100, asignal generator 202, the fiber 160, a coupler 210, a second fiber 211,a splitter 212, a third fiber 213, a fourth fiber 216, a filter 214, anoptical-electrical (OE) converter 218, an analog-to-digital (A/D)converter 222, a signal processor 224, a processor 230, and adigital-to-analog (D/A) converter 234, configured as shown in FIG. 6.The WDM laser transmitter 100 may comprise the laser dice 120, the firstchannels 140, the AWG 150, the second channel 155, which may besubstantially the same as described above. The remaining components ofthe WDM laser transmitter 100 are further described below.

In an embodiment, the signal generator 202 may be an electrical waveformgenerator and may be arranged so as to superpose a pilot signal onto theoutput of an individual laser of the laser dice. The signal generator202 may generate a single pilot signal for all or a subset of all of thelasers in the laser dice 120. The superposition of the pilot signal ontothe output of the laser of the laser dice 120 may facilitate subsequentdistinction of the output of that laser from among a plurality of laserwaveforms. In other embodiments, the pilot signal may be referred to aseither a pilot tone or as dither. In an embodiment, the pilot signal maybe a low-frequency alternating current (AC) sine wave. The frequency ofthe pilot waveform may be lower than the frequencies of the output ofthe laser transmitter, such as about one thousandth of the frequency ofthe output of the laser transmitter, about one millionth of thefrequency of the output of the laser transmitter, or any other fractionof the frequency of the output of the laser transmitter. In anembodiment, the relationship of the amplitude of the pilot signal to theaverage power of the laser transmitter combined output may be referredto as modulation depth (MD). The MD may be selected such that it is lessthan the output of the laser transmitter, such as about one hundredth ofthe average power of the laser transmitter output, about one thousandthof the average power of the laser transmitter output, or any otherfraction of the power of the laser transmitter output. The values ofboth the MD and frequency of the pilot signal may be selected such thatthey minimize interference with the output of the WDM laser transmitter100.

In an embodiment, the output of the WDM laser transmitter 100 may bedirected into the fiber 160. The coupler 210 may be arranged to draw aportion of the signal from the fiber 160 and direct it into the secondfiber 211. In an embodiment, the coupler 210 may be an active coupler ora passive coupler, and may be one of an optical splitter, a y-coupler, astar coupler, a tree coupler, or any other suitable coupler. Thesplitter 212 may be arranged to divide the signal from the fiber 211into two signals: a first signal directed into the third fiber 213 and asecond signal directed into the fourth fiber 216. In an embodiment, thesplitter 212 may be of the dual prism type, the half-silvered mirrortype, the dichroic mirror type, the spliced optical fiber type, or othersuitable splitter.

The third fiber 213 may carry the first signal to the filter 214. Thefilter 214 may modify, alter, or delay the first signal relative to thesecond signal. In an embodiment, the filter 214 may be one of aFabry-Perot interferometer, a Gires-Tournois interferometer, or othersuitable filter, and may be air-spaced, ring-spaced, solid, or of otherconfiguration. In some instances, the filter 214 may be referred to asan etalon. In an embodiment, the filter 214 may be a 50 GHz etalon, a100 GHz etalon, or an etalon of other suitable frequency range orspacing.

The OE converter 218 may receive the second signal, also referred to asa reference signal, from the fourth fiber 216 and the first signal fromthe filter 214. The OE converter 218 may use an optical-to-electricalconversion process to convert the first and second signals from opticalsignals to electrical signals. In an embodiment, the OE converter 218may be a photodiode (PD), light-dependent resistor (LDR), areverse-biased light-emitting diode (LED), a photovoltaic cell, or othersuitable optical-to-electrical converter.

The A/D converter 222 may receive the first signal and the referencesignal from the OE converter 218. The A/D converter 222 may convert thefirst signal and the reference signal from analog signals to digitalsignals. A/D converters 222 are well known in the art, and any suitableA/D converter may be used herein.

The signal processor 224 may receive the first signal and the referencesignal from the A/D converter 222. Each of these two signals may then beprocessed by the signal processor 224, which converts them from the timedomain to the frequency domain. For example, the signal processor 224may implement a Fourier transform, a Fast Fourier transform (FFT), orany other suitable form of time domain to frequency domain processing.

The processor 230 may then process the signal data to facilitatewavelength locking of an individual laser of the laser dice 120. In anembodiment, the signal data may be represented mathematically by thefollowing expressions:

$\begin{matrix}{{F_{s}\left( \omega_{p} \right)} = {\sum\limits_{t}{{V_{s}(t)} \cdot {\mathbb{e}}^{{- {\mathbb{i}\omega}_{p}}t}}}} \\{{F_{r}\left( \omega_{p} \right)} = {\sum\limits_{t}{{V_{r}(t)} \cdot {\mathbb{e}}^{{- {\mathbb{i}}}\;\omega_{p}t}}}}\end{matrix}$where F_(s) is a function of the first signal and F_(r) is a function ofthe reference signal. F_(s) and F_(r) may represent the frequency domainwaveforms that are detected by the OE converter 218, where ω_(p) is thefrequency of the pilot signal, ω_(c) is frequency of the laser to bewavelength locked, V_(s)(t) is the time-domain waveform of the firstsignal, and V_(r)(t) is the time-domain waveform of the referencesignal.

Additional terms may be employed to represent aspects of the wavelengthlocking system 200. For example, P may be used to represent the power ofthe optical output signal of the laser to be locked, ΔP may representthe MD of the pilot signal, and I_(et) may represent the transmissionfunction of the filter 214. Also, Δω_(a) may represent adiabatic chirp,and Δω_(th) may represent thermal chirp, both of which may be introducedby the pilot signal. The term φ_(th) may be used to represent the phasedelay of the thermal chirp relative to the adiabatic chirp, and I′_(et)may represent the first derivative of I_(et) with respect to frequency.Given these definitions, a ratio α of the first signal and the referencesignal, e.g.

${\alpha = \frac{F_{s}\left( \omega_{p} \right)}{F_{r}\left( \omega_{p} \right)}},$may be derived. In an embodiment, the following expressions mayrepresent the real and imaginary components of α:

$\begin{matrix}{I = {{Re}\left\lbrack \frac{F_{s}\left( \omega_{p} \right)}{F_{r}\left( \omega_{p} \right)} \right\rbrack}} \\{= {{I_{et}\left( \omega_{c} \right)} + {\frac{P}{\Delta\; P} \cdot {I_{et}^{\prime}\left( \omega_{c} \right)} \cdot \left( {{\Delta\omega}_{a} + {{\Delta\omega}_{th} \cdot {\cos\left( \phi_{th} \right)}}} \right)}}}\end{matrix}$$Q = {{{Im}\left\lbrack \frac{F_{s}\left( \omega_{p} \right)}{F_{r}\left( \omega_{p} \right)} \right\rbrack} = {\frac{P}{\Delta\; P} \cdot {I_{et}^{\prime}\left( \omega_{c} \right)} \cdot {\Delta\omega}_{th} \cdot {\sin\left( \phi_{th} \right)}}}$where I represents the in-phase component of cc, and Q represents thequadrature component of α. The two expressions for I and Q may beapproximations obtained by neglecting the second and higher orderderivatives of the transmission function of the filter 214. In addition,the quadrature component Q of α may be proportional to the firstderivative of the transmission function I′_(et) of the filter 214. Also,the chirp induced by the pilot signal may not affect the shape of thequadrature frequency response. Further, the thermal chirp and therelated phase delay may only contribute to the magnitude of thequadrature frequency response. The quadrature component may be amplifiedby the inverse of the modulation depth, and may thereby provide enhancedsignal detection. The signal characteristics may indicate that thequadrature component may facilitate effective wavelength locking ofoptical laser transmitters.

The characteristics of the quadrature component, as described herein,may provide a locking point that does not experience significantdeviation from the peak of the optical laser transmitter spectral lineas a result of variations in transmitter properties. Variations betweenindividual transmitters may only affect the error signal strength, notthe locking point. In addition, the affect of this variation on theerror signal strength may be minimized by appropriate choice of pilotsignal frequency and MD. In an embodiment, an appropriate pilot signalfrequency may be greater than or equal to about ten kHz and less than orequal to about 500 kHz, or other suitable frequency. In anotherembodiment, an appropriate MD may be about two percent of the outputpower of the average power of the WDM output, about five percent of theoutput power of the average power of the WDM output or other fraction ofthe power of the WDM output. Hence, the processor 230 may determinewhether the optical transmitters 120 are locked onto the appropriatewavelengths. If an optical transmitter 120 is not locked onto theappropriate wavelengths, the processor 230 may generate an appropriateadjustment signal.

The D/A converter 234 may receive the adjustment signal from theprocessor 230. The D/A converter 234 may convert the adjustment signalfrom a digital signal to an analog signal. D/A converters 234 are wellknown in the art, and any suitable D/A converter may be used herein.

In an embodiment, the OE converter 218, the A/D converter 222, and thesignal processor 224 may be arranged to have individual ports orchannels to manage the two separate signals. In another embodiment, thetwo separate signals may be managed by arranging individual component OEconverters 218, A/D converters 222, and/or signal processors 224appropriately for each of the signals. Alternatively or additionally,the OE converter 218, the A/D converter 222, the signal processor 224,the processor 230, and/or the D/A converter 234 may be discretecomponents as shown or may be combined together into a single component.

In an embodiment, the quadrature component of cc may be employed as acomponent of time-domain multiplexing (TDM) to perform wavelengthlocking of multiple individual lasers of the WDM laser transmitter 100.In an embodiment, TDM may be employed when the signal generator 202applies a pilot signal to a first laser optical transmitter n attimeslot t_(n). The wavelength locker may detect the wavelength λ_(n) ofthe laser optical transmitter n, based on recognition of the pilotsignal applied to the laser optical transmitter n and instruct the laseroptical transmitter n to tune its wavelength λ_(n) to a targetwavelength. This may be repeated for the next timeslot t_(n+1) fortransmitter n+1, and so on until all laser optical transmitters havebeen wavelength locked accordingly.

FIG. 7 illustrates one embodiment of a TDM wavelength locking method300. At block 302, the method 300 starts. At block 304, a pilot signalmay be generated, e.g. using the signal generator, and combined with theoutput signal of a laser of the WDM laser transmitter that is to bewavelength locked. At block 306, the combined output signal of WDM lasertransmitter and the pilot signal may be extracted from the fiber by, forexample, a coupler. In embodiments, the extraction may be from thesecond channel or from the fiber. At block 308, the extracted signal maybe split into a first signal and a reference signal, e.g. by a splitter.

At block 310, the first signal may be directed, for example, through athird fiber to a filter, such as an etalon. At block 312, the firstsignal may then be directed to a detector, such as an OE converter. Atblock 314, the first signal and the reference signal may be processedby, for example, an A/D converter. In an embodiment, the A/D convertermay contain a memory (not shown) to temporarily collect and store aquantity of the signal. In an embodiment, the quantity of the storedsignal may be an amount of time sufficient to provide resolution of thefrequency of the pilot signal, for example about one tenth of the periodof the pilot signal, about two tenths of the period of the pilot signal,about seven tenths of the period of the pilot signal, or other suitablefraction of the frequency of the pilot signal.

At block 316, the signal A/D converter output of the first signal andthe signal A/D converter output of the reference signal may undergoadditional processing, such as by a signal processor. In an embodiment,the signal processor may process the first signal and the referencesignal, where processing may include FFT processing of the signals. Thewavelength locking system may thereby obtain the quadrature component ofα. The magnitude and sign of the quadrature component of a may provideinformation of the deviation of the output wavelength of the WDM lasertransmitter from the target wavelength. For example, if the sign of α ispositive and the magnitude of α corresponds to an offset of 3 GHz, theWDM laser transmitter may be instructed to tune its wavelength in thenegative direction 3 GHz.

At block 318, the output of the signal processor may be directed to aprocessor. In an embodiment, the processor may comprise amicroprocessor, a computer, or any other computing device. At block 320,a determination is made regarding the magnitude and direction ofadjustment of the laser wavelength. At block 322, information may besent to the WDM laser transmitter instructing the WDM laser transmitterto tune its wavelength to the appropriate wavelength. In an embodiment,the tuning may employ adjusting the temperature of the laser or othermeans of adjustment to tune the wavelength of the WDM laser transmitterto the target wavelength. At block 324, the wavelength of the WDM lasertransmitter may be considered locked at the target wavelength. At block326, if there are more lasers to be locked, the wavelength lockingsystem may move to the next laser on the laser dice, using the TDMscheme as described herein. At block 328, method 300 may repeat for eachof N lasers in the laser dice. If at block 326, there are no more lasersto be locked, the method 300 may stop at block 330.

FIG. 8 shows the frequency response of the quadrature component underdifferent chirp conditions. The spectra of quadrature components werecalculated at 0.05 GHz, 0.1 GHz, 0.15 GHz, 0.2 GHz, and 0.25 GHz. Thelocation of the transmission peak of the etalon corresponds to thecrossing point at zero amplitude. A deviation from zero represents anoffset from the target wavelength and can be corrected using aconventional control and/or feedback loop.

FIG. 9 further shows quadrature spectra of the same set of lasersrepresented in FIG. 2, with the addition of a reference signalrepresented by the straight horizontal line. The data shown in FIG. 9was calculated using measured spectra of transmitters and 50 GHz etalonaccording to the system and method of the disclosure. The demonstrateddeviation from zero at the crossing point of the reference line is about+1 GHz, indicating a sufficient level of accuracy to wavelength lock aWDM laser transmitter effectively.

In an embodiment, the system and method taught herein may be implementedwith off-the-shelf components that may be commercially available. In anembodiment, the wavelength locking system 200 of the present disclosuremay be implemented as a frequency locking system, and some embodimentsmay therefore be described and/or implemented in a frequency domainscenario.

FIG. 10 illustrates a typical, general-purpose computer, suitable forimplementing one or more embodiments of any component disclosed herein.The computer 400 includes a processor 402 (which may be referred to as acentral processor unit or CPU) that is in communication with memorydevices including secondary storage 404, read only memory (ROM) 406,random access memory (RAM) 408, input/output (I/O) devices 410, andnetwork connectivity devices 412. The processor may be implemented asone or more CPU chips, or may be part of one or more applicationspecific integrated circuits (ASICs).

The secondary storage 404 is typically comprised of one or more diskdrives or tape drives and is used for non-volatile storage of data andas an over-flow data storage device if RAM 408 is not large enough tohold all working data. Secondary storage 404 may be used to storeprograms that are loaded into RAM 408 when such programs are selectedfor execution. The ROM 406 is used to store instructions and perhapsdata that are read during program execution. ROM 406 is a non-volatilememory device that typically has a small memory capacity relative to thelarger memory capacity of secondary storage 404. The RAM 408 is used tostore volatile data and perhaps to store instructions. Access to bothROM 406 and RAM 408 is typically faster than access to secondary storage404.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R₁, and an upper limit,R_(u), is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R₁+k*(R_(u)−R₁), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. Use of the term “optionally” with respect to anyelement of a claim means that the element is required, or alternatively,the element is not required, both alternatives being within the scope ofthe claim. Use of broader terms such as comprises, includes, and havingmay be understood to provide support for narrower terms such asconsisting of, consisting essentially of, and comprised substantiallyof. Accordingly, the scope of protection is not limited by thedescription set out above but is defined by the claims that follow, thatscope including all equivalents of the subject matter of the claims.Each and every claim is incorporated as further disclosure into thespecification and the claims are embodiment(s) of the presentdisclosure. The discussion of a reference in the disclosure is not anadmission that it is prior art, especially any reference that has apublication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it may be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a plurality of opticaltransmitters coupled to a fiber; a signal generator coupled to theoptical transmitters and configured to provide a single pilot tone tothe optical transmitters; an optical splitter coupled to the fiber,positioned within a feedback loop between the fiber and the opticaltransmitters, and having a first output and a second output; an etalonfilter positioned within the feedback loop and coupled to the firstoutput; an optical-to-electrical (O/E) converter coupled to the etalonfilter and the first output, wherein the O/E converter is configured toconvert an output of the etalon filter and the second output toelectrical signals; and a processor coupled to the O/E converter,wherein the processor is configured to: compute a quadrature componentas an imaginary component of a frequency-domain output of the etalonfilter divided by the second output in the frequency domain; and adjusta wavelength for each of the optical transmitters to lock thewavelengths based on a magnitude and a sign of the quadrature component.2. The apparatus of claim 1, wherein the optical transmitters producedifferent spectral shapes.
 3. The apparatus of claim 1, wherein theoptical transmitters produce substantially the same spectral shapes. 4.The apparatus of claim 1, wherein the pilot tone is provided to theoptical transmitters using time-division multiplexing.
 5. The apparatusof claim 1 further comprising an analog-to-digital (A/D) converterpositioned within the feedback loop between the O/E converter and theprocessor.
 6. The apparatus of claim 5 further comprising a Fast FourierTransform (FFT) processor positioned within the feedback loop betweenthe A/D converter and the processor.
 7. The apparatus of claim 6 furthercomprising a digital-to-analog (D/A) converter positioned within thefeedback loop between the processor and the optical transmitters.
 8. Theapparatus of claim 7, wherein the FFT processor is configured to:compute the frequency-domain output of the etalon filter as a firstFourier transform of the output of the etalon filter; and compute thesecond output in the frequency domain as a second Fourier transform ofthe second output.
 9. The apparatus of claim 8, wherein any deviation ofthe quadrature component from zero represents an offset from a targetwavelength.
 10. The apparatus of claim 9, wherein the first Fouriertransform and the second Fourier transform are each fast Fouriertransforms (FFTs).
 11. An apparatus comprising: an optical-electricalconverter configured to: receive a reference optical signal and atransmitted optical signal, wherein the reference optical signal is afiltered apportionment of the transmitted optical signal comprising apilot tone; and convert the reference optical signal and the transmittedoptical signal to electrical signals; and at least one processorconfigured to: calculate a quadrature component based on a first Fouriertransform of the transmitted optical signal and a second Fouriertransform of the reference optical signal, wherein the quadraturecomponent is the imaginary component of the first Fourier transform ofthe transmitted optical signal divided by the second Fourier transformof the reference optical signal; detect a magnitude and a sign of thequadrature component; and wavelength lock the optical signal using theamplitude and the sign of the quadrature term.
 12. The apparatus ofclaim 11, wherein calculating the quadrature component comprises passingthe optical signal through an etalon filter.
 13. The apparatus of claim11, wherein wavelength locking the optical signal comprises changing atleast one parameter of an optical transmitter that produces the opticalsignal.
 14. The apparatus of claim 13, wherein the pilot tone isprovided to the optical transmitters using time-division multiplexing.15. The apparatus of claim 13, wherein the pilot tone is provided to theoptical transmitter using frequency-division multiplexing.
 16. Theapparatus of claim 11, wherein the frequency of the pilot tone is fromabout 1 kilohertz (kHz) to about 500 kHz.
 17. The apparatus of claim 11,wherein any deviation of the quadrature component from zero representsan offset from a target wavelength.
 18. The apparatus of claim 17,wherein the first Fourier transform and the second Fourier transform areeach fast Fourier transforms (FFTs).
 19. A method comprising: splittingan optical signal generated by an optical transmitter into a firstsignal and a reference signal, wherein the optical signal comprises apilot signal and a data signal; filtering the first signal to obtain afiltered first signal; converting the filtered first signal and thereference signal to electrical signals; computing a first Fouriertransform of the filtered first signal; computing a second Fouriertransform of the reference signal; determining a quadrature componentbased on the first Fourier transform of the filtered first signal andthe second Fourier transform of the reference signal, wherein thequadrature component is an imaginary component of the first Fouriertransform of the filtered first signal divided by the second Fouriertransform of the reference signal; and determining whether an outputfrom the optical transmitter needs to be adjusted based on a magnitudeand a sign of the quadrature component.
 20. The method of claim 19,wherein the quadrature component is modeled as${{\frac{P}{\Delta\; P} \cdot {I_{et}^{\prime}\left( \omega_{c} \right)} \cdot \Delta}\;{\omega_{th} \cdot {\sin\left( \phi_{th} \right)}}},$where P is a power of the optical signal, ΔP is a modulation depth ofthe pilot signal, I′_(et) is a first derivative of a transmissionfunction from a filter with respect to frequency, ω_(c) is a frequencyof the optical signal, Δω_(th) is a thermal chirp introduced by thepilot signal, and φ_(th) is a phase delay of the thermal chirp relativeto an adiabatic chirp introduced by the pilot signal.
 21. The method ofclaim 19, wherein any deviation of the quadrature component from zerorepresents an offset from a target wavelength.
 22. The method of claim21, wherein the first Fourier transform and the second Fourier transformare each fast Fourier transforms (FFTs).